The advent of the information age has fueled widespread commercial, governmental, and personal use of computer printers, copiers, and facsimile machines. Although the full spectrum of these devices encompasses a variety of printing technologies, one of the most prevalent forms is thermal ink-jet printing.
Thermal inkjet printing generally entails applying a fixed amount of electrical energy, in the form of an electrical pulse, to a heater located near a small, ink-filled chamber. The heater heats a portion of the ink until it boils and forms an expanding bubble. The expanding bubble exerts increasing pressure on surrounding ink, ultimately expelling or ejecting some ink through a nozzle as a tiny drop. When the drop lands on paper, it forms a tiny dot, or pixel. (Paper, as used herein, refers to any form of print media.)
The heater-chamber-nozzle combination, generally called a pen, is often part of a printhead having several pens. For example, some color inkjet printers include a printhead with four rigidly positioned pens that respectively eject cyan, yellow, magenta, or black ink. These printers not only move or scan the printhead horizontally across the paper, but also move the paper vertically up or down relative to the printhead. Thus, by selectively moving the printhead and paper and selectively ejecting, or firing, ink drops, the printer forms images, such as text and pictures, on the paper.
The present inventors recognized that conventional ink-jet printers (or more generally imaging systems) may exhibit mechanical imperfections that can cause drop-placement errors. For example, mass-produced printheads typically exhibit some degree of pen-to-pen misalignment. The misalignment forces drops to be ejected at different trajectories, which ultimately causes misalignment of printed dots and reduces image quality.
Another imperfection, known as paper-shape variation, refers to variations in the distance between the printhead and the paper. Paper-shape variation generally stems from shallow hills and valleys in the platen that supports the paper and/or from inconsistent contact of the paper with the platen. The significance of the variation stems from the fact that each pen in the printhead ejects its drops at substantially the same speed, or velocity (based on the fixed amount of energy applied to the pen) and ultimately reduces image quality.
One known way to address both pen-misalignment and paper-shape variation is to delay or advance the timing of the fixed electrical pulses that fire the ink drops and thus shift the landing point of the drops. See, for example, U.S. Pat. No. 6,361,137 (Eaton et al.), which is assigned to the same assignee as the present application and incorporated herein by reference. However, corrections with this approach are generally limited by the printing-grid resolution (or precision) of the printer. Thus, for example, in an ink-jet printer with a 2400 dot-per-inch (dpi) resolution, this pulse-shifting method cannot correct for placement errors less than 1/2400th of an inch.